In the processing of radioactive materials, e.g., mining, ore processing, or nuclear fuel manufacture, the presence of airborne particles of radioactive material presents a hazard to the health of the workers involved. Concern for personnel health and safety requires that the concentration of airborne particulates be maintained at or below predetermined, safe levels. These concentration limits have been established for each radioactive isotope likely to be encountered, and federal law requires that the actual concentration levels be monitored.
Several devices are now commercially available which continuously measure the airborne concentration of most of the isotopes of concern. These devices generally fall into one of two classes: those in which the collection filter must be changed manually, and those in which the filter is automatically changed. In both, a radiation detector is positioned in close proximity to the filter on which the sample is collected. It is desirable to locate the detector as close to the filter as possible (e.g., less than one millimeter) to maximize counting efficiency and energy resolution. However, in the instruments currently available, the sample collection area is usually immediately in front of the detector. This means that the air sample which is being drawn through the filter also passes over the face of the detector and, if the detector is close to the filter, it disturbs the flow of air through the filter. It is thus necessary to maintain a greater than optimum distance between the filter and detector to avoid both disturbing the air flow and contaminating the surface of the detector. In another known type of monitor in which the filter is changed automatically, the sample collection site is separated from the detector by using a strip of filter paper which first passes over the collection site and subsequently past the detector. Since regulations require that a four minute sample at a minimum airflow rate be periodically taken, this results in the use of a great deal of expensive filter paper. Additionally, this device is not adapted for taking integrated counts where long-term samples are taken over the same portion of filter paper and periodically detected.